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Makin' (no) waves. In the computer simulation, a slow-moving cloaked sphere leaves no wake at all as it moves downward (inset, left), while the cloak still reduces the turbulence behind the sphere at faster speeds (inset, middle and right).

A Submarine That Doesn't Make Waves

Wading through water can be such a drag. Even streamlined submarines have to fight the pull of the ocean slowing them down. But with the right outerwear, they may be able to zip through the sea as unburdened as a rocket in outer space—and without leaving so much as a ripple of wake.

Researchers have already developed other types of cloaking devices. An invisibility cloak reported a couple of years ago, for example, makes an object disappear by redirecting light around it. Researchers have also developed materials that can cloak objects against sound waves, ocean waves, and even the elastic waves in Earth's crust caused by earthquakes. Now, Yaroslav Urzhumov and David Smith, both metamaterials researchers at Duke University in Durham, North Carolina, have developed the concept of a wake cloak, which would look like a blade-covered hedgehog and could let an object glide through the water without making waves.

Urzhumov explains that anything moving through water is dragging water with it, making it feel heavier and creating turbulence. But if you just run your little finger through the water, the drag is much smaller. The cloak designed by Urzhumov and Smith makes the cloaked object seem like nothing at all, so the water doesn't pull against it.

The proposed cloak would be a mesh of wires or blades, mounted on the surface of the object moving through water. For their model, the researchers chose a sphere, one of the simplest shapes to simulate. The simulated mesh was layered in 10 concentric shells around the sphere, guiding 10 streams of water. The water nearest the sphere needs the most deflection, so these wires or blades would be thickest. The thinner blades on the outside, however, would hardly change the path of the water, giving it a gentle entrance and exit. Micropumps would control the speed of the water in each layer, ensuring that each stream moved near the same speed as its neighbors. This gradual change from the near stillness of the outer layer to the speed of the sphere in the inner layer would prevent the water from dragging on the sphere or itself.

Urzhumov estimates that the cloak on a 10-centimeter-wide sphere could be anywhere from 1 centimeter to 10 centimeters thick. "Generally, thicker cloaks are easier to fabricate, but they also weigh more, so it's a tradeoff that engineers will decide on," he says.

Shuang Zhang, a metamaterials researcher at the University of Birmingham in the United Kingdom, calls the idea "a valuable extension of invisibility from optics to fluid dynamics". For example, he says, "it can be used for ships to dramatically reduce the dragging force from water and therefore to enhance the speed and efficiency."

At the moment, Urzhumov is setting his sights a bit lower. In the model described in an upcoming issue of Physical Review Letters, the computer simulation studied a fully submerged, bullet-sized vessel that travels at crawling speed, just a few millimeters per second. Yet even this has applications as the United States military explores the possibilities of automaton spies that look like birds, insects, and fish. Urzhumov proposes that a cloaked robo-minnow could stealthily investigate an enemy submarine, moving slowly but requiring little energy. As for when the first cloaks could hit the water, he speculates that it will take at least 5 years of basic research and development to get a working prototype.

However, he and Smith suggest that a different sort of cloak, made specifically to reduce the drag rather than the entire wake, might be easier to make and could be scaled up to fuel-efficient dream boats. "This is a terribly interesting question that someone will answer soon," Urzhumov says.